Java程序辅导

1Socket Programming
Last Modified: 
2/8/2004 8:30:45 AM
Rohan Murty
Hitesh Ballani
Slides adapted from Prof. Matthews’ slides from 2003SP
Socket programming
Socket API
• introduced in BSD4.1 
UNIX, 1981
• Sockets are explicitly 
created, used, released by 
applications
• client/server paradigm 
• two types of transport 
service via socket API: 
– unreliable datagram 
– reliable, byte stream-
oriented 
a host-local, application-
created/owned, 
OS-controlled interface (a 
“door”) into which
application process can 
both send and 
receive messages to/from 
another (remote or 
local) application process
socket
Goal: learn how to build client/server application 
that communicate using sockets
Sockets
Socket: a door between application 
process and end-end-transport protocol 
(UCP or TCP)
process
kernel
buffers,
variables
socket
controlled by
application
developer
controlled by
operating
system
host or
server
process
kernel
buffers,
variables
socket
controlled by
application
developer
controlled by
operating
system
host or
server
internet
Languages and Platforms
Socket API is available for many languages 
on many platforms:
• C, Java, Perl, Python,… 
• *nix, Windows,…
Socket Programs written in any language 
and running on any platform can 
communicate with each other!
Writing communicating programs in different 
languages is a good exercise
2Decisions
• Before you go to write socket code, decide
– Do you want a TCP-style reliable,  full duplex, 
connection oriented channel? Or do you want 
a UDP-style, unreliable, message oriented 
channel?
– Will the code you are writing be the client or 
the server? 
• Client:  you assume that there is a process already 
running on another machines that you need to 
connect to.
• Server:  you will just start up and wait to be 
contacted
Socket programming with TCP
Client must contact server
• server process must 
first be running
• server must have 
created socket (door) 
that welcomes client’s 
contact
Client contacts server by:
• creating client-local 
TCP socket
• specifying IP address, 
port number of server 
process
• When client creates socket: 
client TCP establishes 
connection to server TCP
• When contacted by client, 
server TCP creates new 
socket for server process to 
communicate with client
– Frees up incoming port
– allows server to talk with 
multiple clients
TCP provides reliable, in-order
transfer of bytes (“pipe”) 
between client and server
application viewpoint
Pseudo code TCP client
• Create socket, connectSocket
• Do an active connect specifying the IP 
address and port number of server 
• Read and Write Data Into 
connectSocket to Communicate with 
server
• Close connectSocket
Pseudo code TCP server
• Create socket (serverSocket)
• Bind socket to a specific port where clients can contact 
you 
• Register with the kernel your willingness to listen that on 
socket for client to contact you 
• Loop
Accept new connection (connectSocket)
Read and Write Data Into connectSocket to 
Communicate with client
Close connectSocket
End Loop
• Close serverSocket
3Example: Java client (TCP)
import java.io.*; 
import java.net.*; 
class TCPClient { 
public static void main(String argv[]) throws Exception 
{ 
String sentence; 
String modifiedSentence; 
BufferedReader inFromUser = 
new BufferedReader(new InputStreamReader(System.in)); 
Socket clientSocket = new Socket("hostname", 6789); 
DataOutputStream outToServer = 
new DataOutputStream(clientSocket.getOutputStream());
Create
input stream
Create 
client socket, 
connect to server
Create
output stream
attached to socket
Example: Java client (TCP), cont.
BufferedReader inFromServer = 
new BufferedReader(new
InputStreamReader(clientSocket.getInputStream())); 
sentence = inFromUser.readLine(); 
outToServer.writeBytes(sentence + '\n'); 
modifiedSentence = inFromServer.readLine(); 
System.out.println("FROM SERVER: " + modifiedSentence); 
clientSocket.close(); 
} 
}
Create
input stream
attached to socket
Send line
to server
Read line
from server
Example: Java server (TCP)
import java.io.*; 
import java.net.*; 
class TCPServer { 
public static void main(String argv[]) throws Exception 
{ 
String clientSentence; 
String capitalizedSentence; 
ServerSocket welcomeSocket = new ServerSocket(6789); 
while(true) { 
Socket connectionSocket = welcomeSocket.accept(); 
BufferedReader inFromClient = 
new BufferedReader(new
InputStreamReader(connectionSocket.getInputStream())); 
Create
welcoming socket
at port 6789
Wait, on welcoming
socket for contact
by client
Create input
stream, attached 
to socket
Example: Java server (TCP), cont
DataOutputStream  outToClient = 
new DataOutputStream(connectionSocket.getOutputStream());
clientSentence = inFromClient.readLine(); 
capitalizedSentence = clientSentence.toUpperCase() + '\n'; 
outToClient.writeBytes(capitalizedSentence); 
} 
} 
}
Read in  line
from socket
Create output
stream, attached 
to socket
Write out line
to socket
End of while loop,
loop back and wait for
another client connection
4Client/server socket interaction: TCP 
(Java)
wait for incoming
connection request
connectionSocket =
welcomeSocket.accept()
create socket,
port=x, for
incoming request:
welcomeSocket = 
ServerSocket()
create socket,
connect to hostid, port=x
clientSocket = 
Socket()
close
connectionSocket
read reply from
clientSocket
close
clientSocket
Server (running on hostid) Client
send request using
clientSocketread request from
connectionSocket
write reply to
connectionSocket
TCP 
connection setup
Queues
• We just saw a simple example, with one socket on the 
server handling incoming connections
• While the server socket is busy, incoming connections 
are stored in a queue until it can accept them
• Most systems maintain a queue length between 5 and 
50
• Once the queue fills up, further incoming connections 
are refused until space in the queue opens up
• This is a problem in a situation where our server has to 
handle many concurrent incoming connections. 
Example: HTTP servers
– Solution? Use concurrency
Concurrent TCP Servers
• Benefit comes in ability to hand off processing to another 
process 
– Parent process creates the “door bell” or “welcome” 
socket on well-known port and waits for clients to 
request connection
– When a client does connect, fork off a child process to 
handle that connection so that parent process can 
return to waiting for connections as soon as possible
• Multithreaded server: same idea, just spawn off another 
thread rather than a full process
– Threadpools?
Threadpools
ClientserverSocket
Thread
Pool
Client
1
2 2.1
3
Worker Threads
5Socket programming with UDP
UDP: very different mindset than 
TCP
• no connection just 
independent messages sent 
• no handshaking
• sender explicitly attaches IP 
address and port of 
destination
• server must extract IP 
address, port of sender from 
received datagram to know 
who to respond to 
UDP: transmitted data may be 
received out of order, or lost
application viewpoint
UDP provides unreliable transfer
of groups of bytes (“datagrams”)
between client and server
Pseudo code UDP server
• Create socket
• Bind socket to a specific port where clients 
can contact you 
• Loop
(Receive UDP Message from client x)+
(Send UDP Reply to client x)*
• Close Socket
Pseudo code UDP client
• Create socket
• Loop
(Send Message To Well-known port of 
server)+
(Receive Message From Server)
• Close Socket
Example: Java client (UDP)
import java.io.*; 
import java.net.*; 
class UDPClient { 
public static void main(String args[]) throws Exception 
{ 
BufferedReader inFromUser = 
new BufferedReader(new InputStreamReader(System.in)); 
DatagramSocket clientSocket = new DatagramSocket(); 
InetAddress IPAddress = InetAddress.getByName("hostname"); 
byte[] sendData = new byte[1024]; 
byte[] receiveData = new byte[1024]; 
String sentence = inFromUser.readLine(); 
sendData = sentence.getBytes();
Create
input stream
Create 
client socket
Translate
hostname to IP 
address using DNS
6Example: Java client (UDP), cont.
DatagramPacket sendPacket = 
new DatagramPacket(sendData, sendData.length, IPAddress, 9876); 
clientSocket.send(sendPacket); 
DatagramPacket receivePacket = 
new DatagramPacket(receiveData, receiveData.length); 
clientSocket.receive(receivePacket); 
String modifiedSentence = 
new String(receivePacket.getData()); 
System.out.println("FROM SERVER:" + modifiedSentence); 
clientSocket.close(); 
} 
}
Create datagram with 
data-to-send,
length, IP addr, port
Send datagram
to server
Read datagram
from server
Example: Java server (UDP)
import java.io.*; 
import java.net.*; 
class UDPServer { 
public static void main(String args[]) throws Exception 
{ 
DatagramSocket serverSocket = new DatagramSocket(9876); 
byte[] receiveData = new byte[1024]; 
byte[] sendData  = new byte[1024]; 
while(true) 
{ 
DatagramPacket receivePacket = 
new DatagramPacket(receiveData, receiveData.length); 
serverSocket.receive(receivePacket);
Create
datagram socket
at port 9876
Create space for
received datagram
Receive
datagram
Example: Java server (UDP), cont
String sentence = new String(receivePacket.getData()); 
InetAddress IPAddress = receivePacket.getAddress(); 
int port = receivePacket.getPort(); 
String capitalizedSentence = sentence.toUpperCase(); 
sendData = capitalizedSentence.getBytes(); 
DatagramPacket sendPacket = 
new DatagramPacket(sendData, sendData.length, IPAddress, 
port); 
serverSocket.send(sendPacket); 
} 
} 
}
Get IP addr
port #, of
sender
Write out 
datagram
to socket
End of while loop,
loop back and wait for
another datagram
Create datagram
to send to client
Client/server socket interaction: UDP
close
clientSocket
Server (running on hostid)
read reply from
clientSocket
create socket,
clientSocket = 
DatagramSocket()
Client
Create, address (hostid, port=x,
send datagram request 
using clientSocket
create socket,
port=x, for
incoming request:
serverSocket = 
DatagramSocket()
read request from
serverSocket
write reply to
serverSocket
specifying client
host address,
port umber
7UDP Server vs Client
• Server has a well-known port number
• Client initiates contact with the server
• Less difference between server and client 
code than in TCP
– Both client and server bind to a UDP socket 
– Not accept for server and connect for client
• Client send to the well-known server port; 
server extracts the client’s address from 
the datagram it receives
TCP vs UDP
• TCP can use read/write (or recv/send) and 
source and destination are implied by the 
connection; UDP must specify destination 
for each datagram
– Sendto, recvfrm include address of other 
party
• TCP server and client code look quite 
different; UDP server and client code vary 
mostly in who sends first
Byte ordering
• Big Endian byte-order
The byte order for the TCP/IP protocol suite is 
big endian.
Byte-Order Transformation
8Some Definitions
• Internet Address Structure
struct in_addr
{
in_addr_t s_addr;
};
in_addr_t is defined as a long on linux machines, 
implying 32 bit addresses!
Socket address structure
Address Transformation Socket Types
9Server Client1. Create transport 
endpoint for incoming
connection request: socket()
2. Assign transport
endpoint an 
address: bind( )
5. Wait for a packet
to arrive: read ( )
6. Formulate reply (if any)
and send: write( )
7. Release transport
endpoint: close( )
1. Create transport 
endpoint: socket( )
2. Assign transport
endpoint an 
address (optional):
bind( )
3. Determine address
of server
4. Formulate message
and send: write ( )
6. Release transport
endpoint: close( )
5. Wait for packet
to arrive: read( )
3. Announce willing
to accept connections:
listen( )
4. Block and Wait
for incoming request: 
accept( )
4. Connect to server:
connect( )
C
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 SER
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Connectionless 
Service (UDP)
Server
Client
1. Create transport 
endpoint: socket()
2. Assign transport
endpoint an 
address: bind()
3. Wait for a packet
to arrive: recvfrom()
4. Formulate reply (if any)
and send: sendto()
5. Release transport
endpoint: close()
1. Create transport 
endpoint: socket()
2. Assign transport
endpoint an 
address (optional):
bind()
3. Determine address
of server
4. Formulate message
and send: sendto()
6. Release transport
endpoint: close()
5. Wait for packet
to arrive: recvfrom()
Procedures That Implement The Socket API
Creating and Deleting Sockets
• fd=socket(protofamily, type, 
protocol)
Creates a new socket. Returns a file descriptor 
(fd). Must specify:
• the protocol family (e.g. TCP/IP)
• the type of service (e.g. STREAM or DGRAM)
• the protocol (e.g. TCP or UDP)
• close(fd)
Deletes socket.  
For connected STREAM sockets, sends EOF to 
close connection.
Procedures That Implement The Socket API
Putting Servers “on the Air”
• bind(fd,laddress,laddresslen)
Used by server to establish port to listen on.
When server has >1 IP addrs, can specify 
“IF_ANY”, or a specific one
• listen (fd, queuesize)
Used by connection-oriented servers only, to put 
server “on the air”
Queuesize parameter: how many pending 
connections can be waiting
10
(cont …)
• afd = accept (lfd, caddress, caddresslen)
Used by connection-oriented servers to accept 
one new connection
• There must already be a listening socket (lfd)
• Returns afd, a new socket for the new connection, 
and
• The address of the caller (e.g. for security, log 
keeping. etc.)
Procedures That Implement The Socket API
How Clients Communicate with Servers?
• connect (fd, saddress, saddreslen)
Used by connection-oriented clients to connect to server
• There must already be a socket bound to a 
connection-oriented service on the fd
• There must already be a listening socket on the 
server
• You pass in the address (IP address, and port 
number) of the server.
Used by connectionless clients to specify a “default 
send to address”
• Subsequent “sends” don’t have to specify a 
destination address
Procedures That Implement The Socket API
How Clients Communicate with Servers? 
(TCP)
• int write (fd, data, length)
Used  to send data
• write is the “normal” write function; can be used with 
both files and sockets
• int read (fd, data,length)
Used  to receive data… parameters are similar!
NOTE : both functions can return a value less than 
the length
Procedures That Implement The Socket API
How Clients Communicate with Servers(UDP)
• int sendto (fd, data, length, flags, destaddress,
addresslen) 
Used  to send data.
• Connectionless socket, so we need to specify the 
dest address 
• int 
recvfrom(fd,data,length,flags,srcaddress,addresslen) 
Used  to receive data… parameters are similar, but in 
reverse
11
Concurrent Server: TCP (C/C++)
create socket,
port=x, for
incoming request:
socket(),bind(),listen()
Server (running on hostid) Client
Non-blocking I/O
• By default, accept(), recv(), etc block 
until there’s input
• What if you want to do something else 
while you’re waiting?
• We can set a socket to not block (i.e. if 
there’s no input an error will be returned)
• … or, we can tell the kernel to let us know 
when a socket is ready, and deal with it 
only then
non-blocking/select
• The host uses select() to get the kernel 
to tell it when the peer has sent a 
message that can be recv()’d
• Can specify multiple sockets on which to 
wait
-- select returns when one or more sockets 
are ready 
-- operation can time out !
Java vs C
• Java hides more of the details 
– new ServerSocket of Java = socket, bind and 
listen of C
– new Socket hides the getByName (or 
gethostbyname) of C; Unable to hide this in 
the UDP case though
– Socket API first in C for BSD; more options 
and choices exposed by the interface than in 
Java ?
12
PROJECT 1 : BASIC SOCKETS
AIM:  Write a program (referred to as the IP box) that 
opens four sockets, two TCP and two UDP 
2 TCP SOCKETS :
1. A  receive-config socket : IP BOX acts as a Server (must  
be bound to a port you have to find, and the interface IP 
address) 
2. A send-config socket : IP BOX acts a reciever
(CONT …)
• 2 UDP SOCKETS
• App -- acts as the interface between the IP layer 
and the application
• Iface – represents the network interface  
• Both must be bound to an used port and the 
interface address
IP BOX OPERATION
• Send-config sockets connects to the Test Box and sends 
a “ready-to-test” command
• The Test Box then connects to recv-config socket and 
send a ‘\n’ terminated command which must be echoed
• The Test Box then sends UDP packets to app and iface 
sockets which must be echoed (Note : If the Test Box 
does not receive your echo, it retransmits the packet)
(cont …)
• On receiving both the echoes, the Test Box sends a 
“send-stat” command to the send-config  socket
• The IP box sends a “list-of-stats”
• The Test Box then sends an exit message ( during final 
test, this will have a 40 character hex string representing 
a hashed timestamp, which your program must 
RECORD!)